CHAPTER IX 

 THE NERVOUS SYSTEM 



The nervous system is concerned with the perception of stimuli 

 by receptors, the coordination of stimuli, and the conduction of 

 nervous impulses to effectors. Receptors may be simple, unmodi- 

 fied nerve endings, such as occur in the lower layers of human 

 epidermis, or nerve endings, sometimes highly specialized and 

 intimately related with nonnervous structures, the whole form- 

 ing a sense organ, such as the eye or the ear. Responses to 

 sensory stimulation are manifested in movements of muscles or 

 secretory activity of glands, both of which are the usual effectors. 

 The coordination of the incoming sensory impulses and the 

 responses is the function of nerve centers, such as the brain and 

 the spinal cord. It should be noted that a chemical method of 

 coordination also exists, examples of which were discussed in 

 connection with the secretion of digestive fluids and of which 

 further examples will be considered in the following chapter, 

 dealing with the endocrine organs. Sensation is something 

 produced in the central nervous system, dependent upon the 

 nature of the connections of the nerve fibers carrying the afferent 

 impulses. Afferent nerve fibers normally are stimulated by 

 their receptors, but if a given afferent nerve is stimulated at any 

 point the sensation is always the same. We know the nature of 

 sensations in animals other than Man only by comparing their 

 responses with our own, under similar conditions. Since sensa- 

 tions are entirely subjective, there must be considerable variation 

 in both quantity and quality of sensations produced by the same 

 sensory stimulation in different individuals. 



Reduced to simple terms, the functions of the nervous system 

 and sense organs are: (1) to bring about in the organism appro- 

 priate responses to external stimuli, and (2) to coordinate the 

 functional activity of different parts of the body by nerve path- 

 ways. Irritability and conductivity, two properties manifested 

 by all forms of protoplasm, are highly developed in nervous tissue, 

 and all nervous phenomena are based primarily on these two 



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